The present invention relates to a transducer arrangement for screed control, and more specifically, to a screed control system of a screed head carried by a machine having laser receivers receiving actual elevational positions of the ends of the screed head from an external laser transmitter, and a transducer arrangement providing relative elevational positions of the ends of the screed head to each other.
In using a screed machine for screeding applications, typically, a hydraulic cylinder is connected at each end of a screed head to raise or lower each end of the screed head independently. Accordingly, the two sides of the screed head must be controlled. This is done presently by attaching a pair of laser receivers, one to each side, to the screed head. The elevation feedback provided by the pair of laser receivers drives the hydraulics controlling the elevation of each side of the screed head.
The elevational feedback of each end of the screed head is based upon the pair of laser transmitter detecting a rotating reference beam of laser light that defines a reference plane. This reference plane, emitted by a laser transmission, thus indicates a designated elevation. During a screeding operation, each of the receivers provides elevation feedback to drive the hydraulics controlling the elevation of each side of the screed head in order to maintain the elevational reference.
A problem may arise, however, if the reception of the elevational reference by one the receivers is blocked or interrupted by something of an appreciable height, such as, for example, a support column in a building, or is disrupted. When a blockage or interruption occurs, there is a need to maintain the relative elevation of the ends of the screed head until the elevational reference can be reacquired by both receivers. There is also a need to be able to pull the screed head along a straight path, while maintaining the chosen thickness of the layer and matching forms or existing surfaces during the screeding operation.
One approach to this problem is to set up two external laser transmitters at the same elevation on opposite sides of the screed head. In this way, if a column blocks one of the laser transmitters, the other external laser transmitter is likely to be illuminating the laser receivers at the ends of the screed head, thereby compensating for the interruption. Essentially, the prior art method is to eliminate all blind spots around the receivers. However, this prior art method adds an additional cost in providing and setting up the second external laser transmitter in order to eliminate the possibility of a column block situation.
Another approach to this problem is to use a gravity-based cross slope sensor, which detects the angular shifts of the screed head as the screed head tilts up and down. Additionally, the gravity-based cross slope sensor may be used as a reference for set up and control in a super flat, or plumb, floor application. Accordingly, when both sides of the screed head are within the appropriate dead band, the desired grade of the cross slope sensor is measured and stored in memory of the screed head""s control system. When one laser receiver loses reception of the elevational reference, the cross slope sensor detects the height of the interrupted receiver side of the screed head relative to its uninterrupted receiver side. That is, the cross slope sensor provides a relative measurement of the interrupted laser receiver which, when coupled with the absolute measurement of the uninterrupted laser receiver, provides an estimate of the absolute position of the interrupted laser receive. The control system of the screed head uses the provided absolute and estimated absolute positions to control the elevation of ends of the screed head. However, several disadvantages exist in the cross slope sensor arrangement described above.
First, the gravity-based cross slope sensor is vulnerable to accelerations along its sensitive axis, resulting in miscalculations of the screed head""s slope. In screeding operations, it is quite common for the operator to xe2x80x98side shiftxe2x80x99 the screed head around columns as he pulls the screed head back. Since the sensitive axis of the gravity-based cross slope sensor is parallel to the length of the screed head, this side shifting can cause noticeable acceleration along the sensitive axis of measurement, thus dramatically affecting the feedback of the cross slope sensor. Second, harmonics of the machine boom carrying the screed head, which do not cause significant enough elevation shifts to be seen in the laser receivers at both ends of the screed head, are detectable by gravity-base cross slope sensor since it measures acceleration and not machine movement. Third, in order to reduce the effects of noise and to compensate for some of the low frequency harmonics of the machine vibration, considerable low pass filtering of the cross slope sensor is required. The use of low pass filters on the output of the gravity-based cross slope sensor adds an inherent time lag to the system, which degrades the bandwidth performance of the blocked side. Finally, separate control gains for the cross slope sensor are used to compensate for the time lag. Additionally, changes in loop gain are required to accommodate angular and positional feedback.
Therefore, there is a need for providing a screed control system that does not require setting up a second external laser transmitter in order to maintain the elevation of the ends of the screed head in a block receiver situation.
There is also a need for providing a screed control system that does not require a gravity-based cross slope sensors to maintain the relative elevation of the blocked receiver end of the screed head until the elevation reference can be reacquired by both receivers.
These needs are met by a transducer arrangement according to the present invention that generates control signals indicating relative positions of the ends of the screed head. The control signals of the present invention can be use by the conventional control circuit in a column block situation, which interrupts the reception of a laser beam, providing an elevational reference, from a laser transmitter by one of the laser receivers, to provide an estimated absolute position of the interrupted receiver side until the column block situation clears. Normally, as pointed out above, absolute measurements are available on both sides of the screed head via the laser receivers. Thus, in the present invention, the key to controlling the screed head in a column blocked situation is that at any given time, at least one absolute measurement for one side of the screed head and one relative elevational measurement from that side of the screed head to the blocked side of the screed head is available to the control system. Accordingly, with the generated control signals from the transducer arrangement of the present invention the control system can maintain a relative elevation position of the interrupted receiver side to the absolute position of the uninterrupted receiver side until both receivers can reacquire the elevational reference of the laser transmitter.
In one aspect, the present invention is a transducer arrangement for generating control signals used by a conventional control circuit, which controls movement of hydraulically controlled ends of a screed head with laser receivers provided at the ends, in a column block situation that interrupts the reception of a laser beam, defining a chosen elevational position, from a laser transmitter by one of the laser receivers so as to maintain the chosen elevational position of each hydraulically controlled end of the screed head, the transducer arrangement comprising a light source mounted on a first mast of the screed head; and a light detection device mounted on a second mast of the screed head, the light detection device includes a plurality of light detectors vertically arranged and associated electronics to provide an output to the conventional control circuit indicating which of the plurality of light detectors is illuminated by light from the light source thereby providing an indication of the relative height of the first and second masts, and therefore of the relative height of the ends of the screed head until the column block situation clears.
In another aspect, the present invention is a method of controlling elevational positions of hydraulically controlled ends of a screed head in relationship to a reference plane, defined by a laser transmitter and detected by laser receivers attached the end of the screed head, in a column block situation that interrupts the reception of a laser beam from the laser transmitter by one of the laser receivers, comprising the steps of emitting light from a light source mounted on a first mast of the screed head; detecting the light with a light detection device mounted on a second mast of the screed head, the light detection device includes a plurality of light detectors arranged in a vertical row; generating an output indicating which of the plurality of light detectors is illuminated by light from the light source; and using the output of the light detection device to maintain a relative height between the first and second mast until the column block situation clears.